Transmit diversity method and system

ABSTRACT

The invention relates to a transmit diversity method for a wireless communication system comprising a transmitting element and at least one receiver, wherein a transmission signal is transmitted from the transmitting element to the at least one receiver in accordance with a weight information determined in response to a feedback information. The feedback information is derived from the response at the at least one receiver to the transmission signal, and is fed back using multiplexed feedback signals. Multiple feedback signal quantization constellations and/or constellation specific feedback subchannels can be used for channel probing, such that the total feedback resolution and robustness can be enhanced, while maintaining low signaling capacity of the feedback channel.

RELATED APPLICATIONS

This is a continuation of PCT/EP9/03440, filed May 19, 1999.

FIELD OF THE INVENTION

The present invention relates to a transmit diversity method and systemfor a wireless communication system, such as the Universal MobileTelecommunications System (UMTS) comprising a transmitting element andat least one receiver.

BACKGROUND OF THE INVENTION

Wideband Code Division Multiple Access (WCDMA) has been chosen as theradio technology for the paired bands of the UMTS. Consequently, WCDMAis the common radio technology standard for third-generation wide-areamobile communications. WCDMA has been designed for high-speed dataservices and, more particularly, Internet-based packet-data offering upto 2 Mbps in indoor environments and over 384 kbps for wide-area.

The WCDMA concept is based on a new channel structure for all layersbuilt on technologies such as packet-data channels and servicemultiplexing. The new concept also includes pilot symbols and atime-slotted structure which has led to the provision of adaptiveantenna arrays which direct antenna beams at users to provide maximumrange and minimum interference. This is also crucial when implementingwideband technology where limited radio spectrum is available.

The uplink capacity of the proposed WCDMA systems can be enhanced byvarious techniques including multi-antenna reception and multi-userdetection or interference cancellation. Techniques that increase thedownlink capacity have not been developed with the same intensity.However, the capacity demand imposed by the projected data services(e.g. Internet) burdens more heavily the downlink channel. Hence, it isimportant to find techniques that improve the capacity of the downlinkchannel.

Bearing in mind the strict complexity requirements of terminals, and thecharacteristics of the downlink channel, the provision of multiplereceive antennas is not a desired solution to the downlink capacityproblem. Therefore, alternative solutions have been proposed suggestingthat multiple antennas or transmit diversity at the base station willincrease downlink capacity with only minor increase of complexity interminal implementation.

According to the WCDMA system, a transmit diversity concept is underconsideration which is mainly focused on the closed-loop (feedback)mode.

FIG. 1 shows an example of such a feedback mode for a downlinktransmission between a base station (BS) 10 and a mobile terminal ormobile station (MS) 20. In particular, the BS 10 comprises two antennasA1 and A2, and the MS 20 is arranged to estimate the channel on thebasis of two transmission signals received from the two antennas A1 andA2. Then, the MS 20 feeds back the discretized channel estimate to theBS. Naturally, it is desired to develope a robust and low-delay feedbacksignaling concept.

In WCDMA, three modes are suggested for the closed-loop concept which isoptimized for two antennas. In the feedback (FB) mode 1 (also referredto as Selective Transmit Diversity (STD)), one bit per time slot is usedto signal the “best” antenna from each terminal. The remainingclosed-loop FB modes 2 and 3 provide a slower feedback link, wherefeedback weights used for controlling the antennas A1 and A2 aremodified after two or four 0.625 ms slots, respectively. In this case,the antennas A1 and A2 are co-phased so that transmitted signals sum upcoherently in the MS 20, to thereby provide the best performance withlow mobility “low multipath” environments.

FIG. 2 shows a table indicating characteristic parameters of the aboveFB modes 1 to 3. In particular, N_(FB) designates the number of feedbackbits per time slot, N_(W) the number of bits per feedback signalingword, Na the number of feedback bits for controlling an amplification orpower at the antennas A1 and A2, and Np the number of feedback bits forcontrolling a phase difference between the antennas A1 and A2. As can begathered from the table of FIG. 2, one bit is fed back per time slot ineach of the FB modes 1 to 3.

In the FB mode 1 (i.e. STD), the bit length of the feedback signalingword is one bit, which leads to an update rate of 1600/s (i.e. an updateis performed at the BS 10 in every time slot). The feedback bit rate is1600 bps and the feedback signaling word is used for controlling thepower supplied to the antennas A1 and A2.

In the FB mode 2, the feedback signaling word comprises two bits, whichleads to an update rate of 800/s, since an update is performed afterboth feedback bits have been received, i.e. after two time slots. Thefeedback-signaling word is only used for controlling the phasedifference between the two antennas A1 and A2.

In the FB mode 3, the bit length of the feedback signaling word is four,such that an update rate of 400/s is obtained, i.e. an update isperformed every four time slots. In particular, one bit of the feedbacksignaling word is used for controlling the amplification (power) at theantennas A1 and A2, and three bits are used for controlling their phasedifference.

FIG. 3A shows a table indicating the feedback power control performed inthe FB mode 1 or STD. Here, the MS 20 has to estimate the antenna withthe smallest path loss. To this effect, the MS 20 estimates the channelpower of all “competing antennas”, and determines the one with thehighest power. The required channel estimates are obtained e.g. from acommon pilot channel transmitted with a known power from each antenna.The table in FIG. 3A shows the relationship between the feedback valueand the power P_(A1) supplied to the antenna A1 and the power P_(A2)supplied to the antenna A2. Accordingly, one of the two antennas A1 andA2 is selected at the BS 10 in response to the feedback signaling value.

It is to be noted that the FB mode 1 may be implemented in an analogmanner in the beam domain. In this case, the MS 20 signals to the BS 10whether to rotate channel symbols transmitted from the antenna A2 by180°. In this case, the BS 10 transmits simultaneously from bothantennas A1 and A2. Thus, the phase difference between the antennas A1and A2 is switched between 0° and 180° in response to the feedbackvalue.

The other FB modes 2 and 3 relate to a feedback concept referred to asTransmission Antenna Array (T×AA), in which the MS 20 transmitsestimated and quantized channel parameters to the BS 10 which thenweights the transmitted signals accordingly.

FIG. 3B shows the feedback control performed in the FB mode 2. In the FBmode 2, only a phase weight feedback value comprising two bits is fedback to the BS 10. The phase difference indicated in the table of FIG.3B defines the phase difference (in degree) between the antennas A1 andA2, which is to be established by the BS 10 in order to obtain anoptimum coherence at the MS 20.

FIG. 3C shows the feedback control of the FB mode 3, wherein one bit,i.e. amplification bit, of the feedback signaling word is used forcontrolling the power of the antennas A1 and A2, and the other threebits, i.e. phase bits, are used for controlling the phase differencebetween the antennas A1 and A2. The left-hand table indicates the powercontrol based on the amplification bit, wherein the power P_(A1) andP_(A2) supplied to the antennas A1 and A2, respectively, is switchedbetween 20% and 80% of a predetermined value. The right-hand table showsthe feedback control based on the three phase bits, wherein the phasedifference can be quantified into eight different phase differencevalues to be established by the BS 10 in order to obtain an optimumcoherence in the MS 20.

As regards the table of FIG. 2, it is to be noted that an equal power isapplied to the antennas A1 and A2 in each case where Na=0. Furthermore,the antennas A1 and A2 are uniquely defined by their respective pilotcodes of the CCPCH (Common Control Physical Channel) of the UMTS. Thederived amplitude and phase applied to the antennas A1 and A2 is calleda weight and the set of weights is grouped into a weight vector.Specifically, the weight vector for the present case of two antennas isgiven by$\underset{\_}{w} = \left\lbrack \frac{\sqrt{P_{A1}}}{\sqrt{P_{A2}} \cdot {\exp \left( {j\quad \pi \quad \Delta \quad {\phi/180}} \right)}} \right\rbrack$

wherein ΔΦ denotes the phase difference (phase weight) fed back to theBS 10. In case the dimension of w becomes larger than two, more than twoantennas, i.e. an antenna array, are required, wherein a directionalantenna is achieved by using relative phases between antennas. Theestimated phase of the feedback signal in the complex plane is then usedfor controlling the transmit direction.

Hence, the current WCDMA transmit diversity feedback concept uses a 2, 4or 8 phase constellation to signal the channel difference to the BS 10.However, the higher channel resolution provided by a higherconstellation order is obtained at the expense of feedback signalingcapacity. Thus, the resolution of the feedback signaling is limited bythe feedback signaling capacity. Furthermore, the current conceptimposes a delay of one or more slots in executing the weight change andthis restricts applicability only to very slow fading channels.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for transmit diversity or transmit beamforming, by means ofwhich the resolution of the feedback signaling can be increased withoutincreasing the feedback signaling capacity.

This object is achieved by a transmit diversity method for a wirelesscommunication system comprising a transmitting element and at least onereceiver, said method comprising the steps of:

transmitting from said transmitting element to said at least onereceiver a transmission signal in accordance with a weight informationdetermined in response to a feedback information;

deriving said feedback information from the response at said at leastone receiver to said transmission signal;

feeding back said feedback information using multiplexed feedbacksignals.

Additionally, the above object is achieved by a transmit diversitysystem for a wireless communication system, comprising:

transmitting means for transmitting a transmission signal from atransmitting element in accordance with a weight information determinedin response to a feedback information; and

at least one receiver for receiving said transmission signal andderiving said feedback information from the response to saidtransmission signal;

wherein said at least one receiver comprises a feedback means forfeeding back said feedback information using multiplexed feedbacksignals.

Furthermore, the above object is achieved by a transmitter for awireless communication system, comprising:

extracting means for extracting a feedback information from a receivedsignal;

transmitting means for transmitting a transmission signal from atransmitting element in accordance with a weight information;

determining means for determining the weight information in response tothe extracted feedback information; and

control means for controlling the determining means so as to determinesaid weight information in accordance with multiplexed feedback signalsused for feeding back said feedback information.

Moreover, the above object is achieved by a receiver for a wirelesscommunication system, comprising:

receiving means for receiving a transmission signal;

deriving means for deriving a feedback information from the response tosaid transmission signal; and

feedback means for feeding back said feedback information usingmultiplexed feedback signals.

Accordingly, the transmit resolution can be enhanced by maintaining thefeedback channel resolution and capacity signaled from the receiver andperforming a suitable feedback filtering at the transmitter inaccordance with the time-varying feedback signal constellation. Thereby,the effective resolution of the total feedback signaling can be improvedwhile maintaining the signaling channel capacity, since the feedbackinformation can be divided and spread over different sets of time slotsin accordance with the time-varying signal constellation, or by usingmultiple different constellations. The filtering is applied to at leasttwo subchannels. The transmitting signal may comprise a probing signalused for channel measurements and channel quantization and aninformation transmitted via the dedicated channel on the basis of thetransmit weights.

According to the invention, multiplexed feedback signals can be used forrepresenting the quantized state of the channel. Thereby, the type,coding, partitioning or allocation of the feedback signals may differ indifferent multiplex subchannels defined by a time division, frequencydivision, or code division multiplexing scheme.

Thus, the weights applied to the antennas A1 and A2 can be demultiplexedfrom the feedback channel and need not be identical with the feedbacksignaling of the current time slot received from the receiver. Inparticular, a multiplex timing can be arranged such that the current FBmodes still can be established. Each subchannel may independently definea basic resolution, and the subchannels may jointly define an increasedresolution. According to the invention, at least two feedbacksubchannels are used. The multiplexed feedback signals are demultiplexedat the transmitting element and then filtered in order to obtain thedesired transmit weights. Thus, a flexible feedback concept is achieved,in which the transmit weights are derived from the feedback signals butneed not match them exactly.

Furthermore, a higher transmit weight resolution and robustness can beachieved e.g. by multiplexing different feedback signals which are to becombined in a suitable way, e.g. by a Finite Impulse Response (FIR)filtering or an Infinite Impulse Response (IIR) filtering, at thetransmitter. The filtering can also take into account the reliability ofthe received feedback signals. Then, the filter can determine theweights based on a higher weighting of the reliable feedback signals.Therefore, the present FB mode 3 can be achieved, since it can beestablished on the basis of e.g. the present FB mode 2 by multiplexingtwo different feedback signals and filtering them suitably. In thiscase, the feedback signaling and the channel estimation can bemaintained, while slightly changing the feedback signal determination.However, no changes are required to the common channels.

The length of the filter impulse response should be matched to thechannel characteristics (e.g. Doppler spread) in the sense that longerfilters can be used when channel changes are slow. The type of filtercan be determined from the received signal or it can be negotiatedbetween the transmitter and the receiver. Furthermore, thedemultiplexing and subsequent filtering can be performed on the feedbacksignal or on the transmit weights to which the feedback signalscorrespond, or both. In particular, gain and phase information can befiltered separately or jointly. The filter can operate as a predictor,so that transmit weights can be predicted based on the availablesmoothed information until the command is transmitted, current weightsand/or previous weights and/or received feedback commands. In addition,the filtering can be linear or non-linear. Furthermore, a robustfiltering, e.g. using a median filtering, can be applied, which ispreferred, since feedback errors may cause “outliers” weights, i.e.erroneous weights due to a wrong index rather than an estimation errorin determining the index/quantization.

Hence, the channel is quantized to a plurality of feedback signalquantization constellations, and each quantized value is transmitted viadifferent multiplexed feedback subchannels. Thereby, a user may usedifferent channel quantization constellations at different quantizationintervals which may possibly overlap. The different quantizationconstellations may be independent, e.g. suitable rotations of eachother, or may be formed in a dependent or hierarchical manner by a setpartitioning, wherein the dependent constellations are jointly used todefine the feedback signal with increasing accuracy (e.g. the first twobits transmitted in a first subchannel may designate a weight quadrant,and the third bit transmitted in a second subchannel may specify one oftwo weight points within the weight quadrant). Furthermore, differentquantization constellations can be provided for different users.

Preferably, the multiplexed feedback signals may comprise a firstfeedback signal having a first constellation and a second feedbacksignal having a second constellation. The first and second feedbacksignals may be transmitted in different time slots and/or by usingdifferent codes.

The first feedback signal may define a first phase weight determined onthe basis of a channel estimate, and the second feedback signal maydefine a second phase weight determined on the basis of a rotatedconstellation. In particular, the second phase weight may be based on arotated channel estimate of the same constellation, or on a rotatedchannel estimate of another constellation, or on the basis of aquantization of the channel estimate to the second (rotated)constellation. The first and second feedback signals may be fed back insuccessive time slots. Moreover, the first feedback signal may define areal part of the weight information, and the second feedback signal maydefine an imaginary part of the weight information.

Alternatively, the first feedback signal may define a first feedbackinformation to be used for updating a first beam of the transmittingelement, and the second feedback signal may define a second feedbackinformation to be used for updating a second beam of the transmittingelement. In this case, the first feedback signal can be fed back duringodd time slots and the second feedback signal during even time slots.The odd and even time slots may be used for controlling the same antenna(when the channel difference is used) or a first antenna and a secondantenna, respectively, in different time instants. In the latter case,the first and second antennas are alternately used as a reference.Controlling both antennas, e.g. by transmitting control commands in analternate manner to the transmitting element, is preferred in caseswhere the effective transmitting power of the controlled antenna can bereduced by the filtering. When both antennas are generally controlled,the effective transmitting power is distribuited evenly and thissimplifies the designs of a provided power amplifier. Another possiblesolution is to use transmit diversity techniques where different usersmay control different antennas.

Furthermore, the first feedback signal may define a quadrant in a 4-PSKconstellation, and the second feedback signal may define a constellationwithin said quadrant defined by said first feedback signal. The secondfeedback signal may define a differential change, a Gray-encodedsub-quadrant, or a combination thereof.

The multiplexed feedback signals may be transmitted by at least twousers having different feedback signal constellations. Thereby, aflexible and readily adaptable transmit diversity system can beachieved. The at least two users may comprise a first set of userscontrolling weights at a first antenna of the transmitting element, anda second set of users controlling weights at a second antenna of saidtransmitting element. In this case, a useful balancing of thetransmitting power between the first and second antennas can beprovided, since some filtering or demultiplexing techniques may resultin lower transmission power requirements at the controlled antenna

Furthermore, the control means provided in the transmitter may comprisea switching means for alternately switching the first feedback signaland the second feedback signal to the determining means. The determiningmeans may be arranged to derive the weight information from the firstand second feedback signal.

Moreover, the control means may be arranged to control the transmittingmeans so as to alternately update a first beam of the transmittingelement by using a first weight information determined on the basis ofthe first feedback signal, and a second beam of the transmitting elementby using a second weight information determined on the basis of thesecond feedback signal.

The transmitting element may be an antenna array. In this case, thefeedback information can be used for controlling the direction oftransmission of the array antenna. The transmission direction may bederived from at least one of the multiplexed feedback signals.Furthermore, the transmission direction may be derived from a phaseestimate obtained from at least one feedback signal.

Furthermore, the deriving means of the receiver may comprise extractingmeans for extracting a probing signal transmitted with a known power,channel estimation means for performing a channel estimation on thebasis of the extracted probing signal, and generating means forgenerating the multiplexed feedback signals on the basis of the channelestimation. The generating means may be arranged to generate the firstand second feedback signal, wherein the feedback means may be arrangedto feed back the first and second feedback signals as the multiplexedfeedback signals. The first and second feedback signals may be fed backalternately by the feedback means, wherein a quantization of thefeedback information is based on the latest channel estimate and anavailable one of the first and second constellation.

Moreover, the generating means may be arranged to generate the firstfeedback signal based on the channel estimate and the second feedbacksignal based on a rotation of the channel estimate by a predeterminedangle. This can be implemented also by quantizing the same channelestimate to two constellations where, in this case, the second one is arotated copy of the first one.

Alternatively, the generating means may be arranged to generate thefirst feedback signal based on a real part of the feedback information,and the second feedback signal based on an imaginary part of thefeedback information.

As a further alternative, extracting means may be arranged toalternately extract a probing signal corresponding to a first beam and aprobing signal corresponding to a second beam, and the generating meansmay be arranged to alternately generate the first feedback signal basedon a channel estimate for the first beam, and the second feedback signalbased on a channel estimate for the second beam.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail on the basis of a preferred embodiment with reference to theaccompanying drawings, in which:

FIG. 1 shows a principle block diagram of a closed-loop transmitdiversity system comprising a base station and a mobile station,

FIG. 2 shows a table indicating characteristic parameters of the FBmodes 1 to 3,

FIGS. 3A to 3C show tables indicating characteristic parameters relatingto the feedback control of the FB modes 1, 2 and 3, respectively,

FIG. 4 shows tables indicating characteristic parameters of the transmitdiversity concept according to a first example of the preferredembodiment of the present invention,

FIG. 5 shows a principle block diagram of a base station and a mobilestation according to the preferred embodiment of the present invention,

FIG. 6 shows a diagram of complex weight parameters according to thefirst example of the preferred embodiment,

FIG. 7 shows tables indicating characteristic parameters of the transmitdiversity concept according to a second example of the preferredembodiment,

FIG. 8 shows a diagram of complex weight parameters according to thesecond example of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, the preferred embodiment of the method and systemaccording to the present invention will be described on the basis of aconnection between the BS 10 and the MS 20 of the UMTS, as shown in FIG.1.

According to the preferred embodiment of the present invention, thefeedback information is transmitted from the MS 20 to the BS 10 using afeedback concept based on time multiplexing. This means that theconstellation of the feedback signals is changed and signaled to the BS10 in different time slots. However, any other multiplex scheme such asfrequency multiplexing or code multiplexing may be used as well in thefeedback channel.

In particular, the feedback signal constellation may be changed withrespect to the coding, type, partitioning or allocation of the feedbackinformation. Thus, with the present time multiplexed feedbacksubchannels, the signaling capacity required in the feedback channel canbe maintained, while the feedback information as such is spread over thetime axes, i.e. transmitted in two or more (sets of) time slots whichmay be allocated according to a predefined rule, known to both the BS 10and the MS 20.

In the following, three examples of the preferred embodiment aredescribed with reference to FIGS. 4 to 8, wherein the feedbackinformation is spread over successive time slots.

FIG. 4 shows two tables indicating a refined mode 2 concept. Accordingto this example, two reference channels, i.e. the channel estimate and arotated channel estimate, are used in the MS 20 in order to derive thefeedback information. Thereby, an 8-phase signaling can be implementedby using the mode 2 feedback signaling, i.e. two feedback bits. Inparticular, a first feedback information relating to the channelestimate is transmitted in two successive time slots, and a secondfeedback information relating to the rotated channel estimate istransmitted in the following two successive time slots. Thus, the wholefeedback information is transmitted in four successive time slots.Accordingly, the phase difference relating to the channel estimate istransmitted in slots S1={1, 2, 5, 6, 9, 10, . . . } defining a firstfeedback subchannel, and the phase difference quantized to the rotatedconstellation is transmitted in slots S2={3, 4, 7, 8, 11, 12, . . . }defining a second feedback subchannel, wherein the rotated channelestimate relates to a 45° rotated channel estimate, assuming a 4-phaseconstellation is used.

Thus, the effective phase differences for the phase bits transmitted inthe slots S1 is indicated by the upper table of FIG. 4, and the phasedifference defined by the phase bits transmitted in the slots S2 isindicated in the lower table of FIG. 4. Accordingly, the phasedifference can be quantized into 8 values while using only two bits offeedback information at a time, as in the FB mode 2. The resultingfeedback resolution obtained by a filtering or demultiplexing operationat the BS 10 corresponds to the FB mode 3, with the exception that aconstant power is used for each of the antennas A1 and A2. Thus, thefeedback resolution can be increased while maintaining the feedbacksignaling capacity of the FB mode 2.

FIG. 5 shows a principle block diagram of the MS 20 and the BS 10according to the preferred embodiment of the present is invention.

According to the FIG. 5, the BS 10 comprises a transceiver (TRX) 11arranged for feeding the two antennas A1 and A2 and connected to anextracting unit 12 provided for extracting the feedback informationtransmitted from the MS 20 via the corresponding feedback channel(s).The extracted feedback information is supplied to a switch 13 which iscontrolled by a timing control unit 15 in accordance with the timingscheme underlying the multiplex scheme of the feedback signalconstellation used by the MS 20. Thereby, a demultiplexing or filteringfunction for extracting the feedback information is provided. In thepresent example, the switch 13 is controlled by the timing control unit15 so as to supply the feedback information relating to the slots S1 toone of its output terminals and the feedback information transmitted inthe slots S2 to the other one of its output terminals.

It is noted that the above demultiplexing or filtering function mayalternatively be achieved by providing filter and demodulating unit or adecoding unit, in case a frequency or, respectively, code multiplexscheme is used.

The output terminals of the switch 13 are connected to respective inputterminals of a weight determination unit 14 which determines a weightsignal on the basis of the tables shown in FIG. 4. In particular, theweight determination unit 14 determines the required phase differencebetween the antennas A1 and A2 by averaging the feedback information ofthe two slot types S1 and S2 received via the respective inputterminals. However, any other combination of the two feedbackinformations may be provided.

The determined weight signal, e.g. phase difference, is supplied to theTRX 11 which performs a corresponding phase control of the antennas A1and A2 to thereby establish the required phase difference leading to anoptimum coherence of the transmission signals in the MS 20.

The MS 20 comprises a transceiver (TRX) 21 for receiving thetransmission signals from the antennas A1 and A2 of the BS 10 via anantenna connected thereto. Furthermore, the TRX 21 is connected to anextracting unit 22 provided for extracting the pilot channel signal andsupplying the extracted pilot channel signal to a channel estimationunit 23 which calculates the required channel estimates. In particular,the channel estimation unit 23 is arranged to calculate the channelestimate and the rotated channel estimate both corresponding to thereceived pilot channel signal. The channel estimation unit 23 outputsthe two channel estimates at respective output terminals thereof whichare connected to corresponding input terminals of a channel differencederiving and quantization unit 24 for deriving a phase difference basedon the channel estimate and the rotated channel estimate obtained fromthe channel estimation unit 23 and performing a correspondingquantization. As already mentioned, the rotated channel estimate isobtained by rotating the channel estimate by an angle of 45°.

Furthermore, a feedback timing unit 25 is provided which controls thephase difference deriving and quantization unit 24 so as to output oneof the phase differences derived from the channel estimate and therotated channel estimate in accordance with the predetermined feedbacktiming. In the present case, the phase difference corresponding to thechannel estimate, i.e. conventional FB mode 2, is outputted during thetime slots S1, and the phase difference corresponding to the rotatedchannel estimate is outputted during the time slots S2. The phasedifferences are supplied as a multiplexed feedback signal to the TRX 21in order to be transmitted via the corresponding feedback channel to theBS 10.

It is to be noted that the transmit diversity concept according to thefirst example of the preferred embodiment is compatible with the knownFB mode 2, in case the BS 10 assumes each feedback information asderived only from the channel estimate which has not been rotated, i.e.the known BS 10 controlled according to the FB mode 2.

In case a frequency or code multiplex feedback scheme is used, thefeedback timing unit 25 may be replaced by a modulating unit or,respectively, a coding unit.

FIG. 6 shows a diagram of the complex weights or end points of theweight vectors used as the feedback information in the first example ofthe preferred embodiment. In particular, the circles in the diagram ofFIG. 6 indicate the weights obtained in the slots S1, i.e. the weight ofthe conventional FB mode 2, and the crosses indicate the additionalweights obtained in the time slots S2. Thus, a phase differencequantization as provided in the FB mode 3 can be obtained withoutincreasing the feedback channel signaling capacity.

FIG. 7 shows a second example of the preferred embodiment, wherein thefeedback resolution of the FB mode 2 is obtained while using only asingle feedback bit. Thus, this example relates to a refined FB mode 1.In particular, the MS 20 performs a continuous measurement or channelestimation, e.g. on the basis of a sliding window, and the phasedifference deriving unit 24 quantizes the phase difference in accordancewith the FB mode 2 phase constellation. In the present case, thefeedback bits for the real and imaginary part of the complex weight,determined by the phase difference, are transmitted in successive slots,e.g. the real part bit in the odd slots used as a first feedbacksubchannel and the imaginary part bit in the even slots used as a secondsubchannel. A corresponding control is performed by the feedback timingunit 25 of the MS 20.

Correspondingly, the timing control unit 15 of the BS 10 controls theswitch 13 so as to supply the successive real and imaginary part of thefeedback information to respective input terminals of the weightdetermination unit 14 which determines the corresponding weight signalsupplied to the TRX 11 in order to establish the required phasedifference.

In case the BS 10 is not controlled in accordance with this time controlscheme, i.e. the current FB mode 1 is used, the conventional control isobtained. If the new timing control is provided, the weightdetermination unit 14 averages over two slots and changes the weightsignal correspondingly.

Thus, an FB mode 2 resolution is obtained with an FB mode 1 feedbackcapacity. Moreover, an antenna verification can be incorporatedseparately for the successive bits, which corresponds to the STDconcept.

Thus, as can be gathered from FIG. 7, the feedback information providedin the odd slots S_(odd) indicates a phase difference of 0° or 180°, andthe feedback information provided in the even slots S_(even) indicates aphase difference of −90° or +90°.

FIG. 8 shows a diagram of the complex weights which can be fed back inthe second example of the preferred embodiment, wherein the crossesindicate the weight information transmitted in the slots S_(even) andthe circles indicate the weights transmitted in the slots S_(odd).

According to a third example of the preferred embodiment, a beamdiversity concept can be adopted by the feedback scheme in order toprovide an enhanced robustness against erroneous signaling. In the thirdexample, it is assumed that a space time coding (STTD) is used at the MS20, wherein encoded channel symbols are divided into two-element blocksand transmitted as b[2n], b[2n+1] and b*[2n+1], b*[2n] from the antennasA1 and A2, respectively, during time instants 2n and 2n+1 using the samespreading code. This simple symbol level orthogonal coding schemedoubles the time diversity, wherein the receiver uses a simple lineardecoding to detect the transmitted symbols. In the present case, twoweight vectors are used, which are a function of the received signaling.In case of the FB mode 1 feedback signaling, the following processing isperformed.

Two beams B1 and B2 are transmitted by the antennas A1 and A2 of the BS10 in each time slot. The update rate of the beams B1 and B2 is 800 Hz,i.e. the TRX 11 is updated every other time slot. In particular, thebeam B1 is modified during odd slots and the beam B2 during even slots,where each weight modification is effective over two time slots, i.e. asliding window weight change is provided. Hence, the extracting unit 22of the MS 20 is arranged to extract the corresponding probing or pilotsignals received from the beams B1 and B2, and to successively supplythem to the channel estimation unit 23. Then the feedback timing unit 25controls the phase difference deriving unit 24 so as to output therespective phase differences at timings corresponding to their allocatedtime slots.

It is to be noted that the filtering function provided by the switchunit 13 and the timing control unit 15 of the BS 10 is not required inthe present case, if the TRX 11 is arranged to determine andcorrespondingly allocate successively received weight signals to theirrespective beams B1 or B2. However, if this is not the case, the timingcontrol unit 15 controls the switch 13 so as to switch the weight signalof the beam B1 (transmitted in an odd slot) to one of its outputterminals and the weight signal of the beam B2 (transmitted in an evenslot) to the other output terminal and the weight determination unit 14determines the corresponding weight signal. In addition, the timingcontrol unit 15 is arranged to control the TRX 11 so as to allocate thereceived weight signal to the corresponding one of the beams B1 and B2.This control feature is indicated by the broken error shown in the block35 diagram of the BS 10 of FIG. 5.

It is to be noted that the above described units of the block diagramshown in FIG. 5 may as well be established as software features of acontrol program controlling a microprocessor such as a CPU provided inthe BS 10 and the MS 20.

Furthermore, any kind of signal set partitioning (e.g. for trelliscodes) may be used to improve the performance. Furthermore, thedifferent feedback signal constellations may be dependent by using aprogressive signaling. For example, a first time slot or subchannel canbe used for feeding back an information indicating a quadrant in a 4-PSKconstellation with higher reliability, and a subsequent second time slotor subchannel can be used for feeding back an information determiningthe constellation within this quadrant. The feedback information of thesecond subchannel may be based on a differential change, a Gray-encodedsub-quadrant, or any combination thereof. Here, the transmit weights canbe changed as soon as the feedback bits specifying the quadrant havearrived at the BS 10, and the refined subquadrant can be adjustedthereafter based on the most recent channel estimate, which was notavailable when the quadrant index was transmitted (e.g. using Grayencoding). Thereby, additional delay caused in the current concept bywaiting for the receipt of all feedback bits can be prevented.Furthermore, abrupt changes (180 degree in case of one bit feedback, 90degrees in case of two bit feedback, and so on), as in the currentconcepts, which cannot be followed by the MS 20 estimating the dedicatedchannel parameters do not occur. Hence, applying the feedbackinformation incrementally not only reduces delay, but also enables moreefficient channel estimation and receiver performance. The feedbackinformation may also refer to the phase difference of successive slots.

Furthermore, the present invention is not limited to two antennas A1 andA2, but can be applied to any multi-antenna transmitter in order toprovide a higher resolution feedback. Moreover, as already mentioned,any kind of multiplex scheme can be used, provided the BS 10 is arrangedto correspondingly filter or select the feedback information.

Furthermore, the present invention may be applied to any wirelesscommunication system comprising a transmit diversity or transmitbeamforming concept used between a transmitting element and at least onereceiver. Therefore, the above description of the preferred embodimentand the accompanying drawings are only intended to illustrate thepresent invention. The preferred embodiment of the invention may varywithin the scope of the attached claims.

In summary, the invention relates to a transmit diversity method for awireless communication system comprising a transmitting element and atleast one receiver, wherein a transmission signal is transmitted fromthe transmitting element to the at least one receiver in accordance witha weight information determined in response to a feedback information.The feedback information is derived from the response at the at leastone receiver to the transmission signal, and is fed back usingmultiplexed feedback signals. Thus, multiple quantization constellationsand/or constellation specific feedback subchannels can be used forchannel probing, such that the total feedback resolution can beenhanced, while maintaining low signaling capacity of the feedbackchannel.

What is claimed is:
 1. A transmit diversity method for a wirelesscommunication system comprising a transmitting element and at least onereceiver, said method comprising the steps of: a) transmitting from saidtransmitting element to said at least one receiver a transmission signalin accordance with a weight information determined in response to afeedback information; b) deriving at said at least one receiver saidfeedback information from the response to said transmission signal usingat least two different quantization constellations; c) feeding back saidfeedback information using multiplexed feedback signals, wherein saidmultiplexed feedback signals comprise a first feedback signal having afirst quantization constellation and a second feedback signal having asecond quantization constellation.
 2. A method according to claim 1,wherein said first and second feedback signals are transmitted indifferent time slots.
 3. A method according to claim 1, wherein saidfirst and second feedback signals are transmitted using different codes.4. A method according to claim 1, wherein said first feedback signaldefines a first weight determined on the basis of a channel estimatequantized to said first constellation, and said second feedback signaldefines a second weight determined on the basis of a channel estimatequantized to said second constellation.
 5. A method according to claim4, wherein said second constellation is a rotated copy of said firstconstellation obtained from said first constellation by multiplying eachelement in the constellation by exp(i*theta).
 6. A method according toclaim 4, wherein said second feedback signal is based on a rotatedchannel estimate quantized to said first constellation.
 7. A methodaccording to claims 1, wherein said first and second feedback signalsare fed back in successive time slots.
 8. A method according to claim 1,wherein said first feedback signal defines a real part of said weightinformation, and said second feedback signal defines an imaginary partof said weight information.
 9. A method according to claim 1, whereinsaid first feedback signal defines a first feedback information to beused for updating a first beam of said transmitting element, and saidsecond feedback signal defines a second feedback information to be usedfor updating a second beam of said transmitting element.
 10. A methodaccording to claim 8, wherein said first feedback signal is fed backduring odd time slots, and said second feedback signal is fed backduring even time slots.
 11. A method according to claim 1, wherein atleast one quantization constellation depends on at least one of thepreviously transmitted quantizations.
 12. A method according to claim11, wherein said first feedback signal defines a quadrant in a 4-PSKconstellation, and said second feedback signal defines a constellationpoint within said quadrant defined by said first feedback signal.
 13. Amethod according to claim 12, wherein said second feedback signaldefines a differential change, a Gray-encoded sub-quadrant, or acombination thereof.
 14. A method according to claim 1, wherein saidmultiplexed feedback signals are transmitted by at least two usershaving different signal constellations.
 15. A method according to claim14, wherein said at least two users comprise a first set of userscontrolling weights at a first antenna of said transmitting element, anda second set of users controlling weights at a second antenna of saidtransmitting element.
 16. A method according to claim 1, wherein saidfeedback information is used for controlling a transmit weight of one oftwo antennas.
 17. A method according to claim 1, wherein said feedbackinformation is used for controlling transmit weights of two antennas.18. A method according to claim 17, wherein control commands forcontrolling said two antennas are transmitted alternately to saidtransmitting element.
 19. A method according to claim 1, wherein saidtransmitting element comprises an antenna array.
 20. A method accordingto claim 19, wherein said feedback information is used for controllingthe direction of transmission of said antenna array.
 21. A methodaccording to claim 20, wherein the direction of transmission is derivedfrom at least one feedback signal.
 22. A method according to claim 21,wherein the direction of transmission is derived from a phase estimateof at least one extracted feedback signal.
 23. A method according toclaim 1, wherein said weight information and/or a direction oftransmission are determined on the basis of a feedback signal filteringoperation.
 24. A method according to claim 23, wherein said filteringoperation comprises a robust filtering, an FIR filtering, an IIRfiltering, a linear filtering, a non-linear filtering, or a smoothingand prediction.
 25. A method according to claim 1, wherein a reliabilityof said multiplexed feedback signals is used for weight determination.26. A method according to claim 23, wherein a transmission filtering isadapted to a transmission channel characteristic and changeddynamically.
 27. A transmit diversity system for a wirelesscommunication system, comprising: a) transmitting means (10) fortransmitting a transmission signal from a transmitting element (A1, A2)in accordance to with a weight information determined in response to afeedback information; and b) at least one receiver (20) for receivingsaid transmission signal and deriving said feedback information from theresponse to said transmission signal using at least two differentquantization constellations; c) wherein said at least one receiver (20)comprises feedback means (24, 25) for feeding back said feedbackinformation using multiplexed feedback signals, wherein said feedbackmeans (24, 25) is arranged to generate a first feedback signal having afirst constellation and a second feedback signal having a secondconstellation.
 28. A system according to claim 27, wherein said firstfeedback signal defines a first phase weight determined on the basis ofa channel estimate, and said second feedback signal defines a secondphase weight determined on the basis of a rotated constellation of saidfirst feedback signal.
 29. A system according to claim 27, wherein saidfirst feedback signal defines a real part of said weight information,and said second feedback signal defines an imaginary part of said weightinformation.
 30. A system according to claim 27, wherein said firstfeedback signal defines a first feedback information to be used by saidtransmitting means (10) for updating a first beam of said transmittingelement (A1, A2), and said second feedback signal defines a secondfeedback information to be used by said transmitting means (10) forupdating a second beam of said transmitting element (A1, A2).
 31. Asystem according to claim 29, wherein said feedback means (24, 25) isarranged to feed back said first feedback signal during odd time slotsand said second feedback signal during even time slots.
 32. Atransmitter for a wireless communication system, comprising: a)extracting means (12) for extracting a feedback information from areceived signal; b) transmitting means (11) for transmitting atransmission signal from a transmitting element (A1, A2) in accordancewith a weight information; c) determining means (14) for determiningsaid weight information in response to said extracted feedbackinformation; and d) control means (13, 15) for controlling saiddetermining means (14) so as to determine said weight information inaccordance with multiplexed feedback signals used for feeding back saidfeedback information, wherein said control means (13, 15) comprises aswitching means (13) for alternately switching a first feedback signalhaving a first constellation and a second feedback signal having asecond constellation to said determining means (14).
 33. A transmitteraccording to claim 32, wherein said determining means (14) is arrangedto derive said weight information from said first and second feedbacksignals.
 34. A transmitter according to claim 32, wherein said controlmeans (13, 15) is arranged to control said transmitting means (11) so asto alternately update a first beam of said transmitting element (A1, A2)by using a first weight information determined on the basis of saidfirst feedback signal, and a second beam of said transmitting element(A1, A2) by using a second weight information determined on the basis ofsaid second feedback signal.
 35. A transmitter according to claim 32,wherein said transmitting element is an antenna array (A1, A2).
 36. Areceiver for a wireless communication system, comprising: a) receivingmeans (21) for receiving a transmission signal; b) deriving means (22,23, 24) for deriving a feedback information from the response to saidtransmission signal using at least two different quantizationconstellations; and c) feedback means (24, 25) for feeding back saidfeedback information using multiplexed feedback signals, wherein saidderiving means (22, 23, 24) comprises extracting means (22) forextracting a probing signal transmitted with a known power, channelestimation means (23) for performing a channel estimation on the basisof said extracted probing signal, and generating means (24) forgenerating said multiplexed feedback signals on the basis of saidchannel estimation, and wherein said generating means (24) is arrangedto generate a first feedback signal having a first constellation and asecond feedback signal having a second constellation, wherein saidfeedback means (24, 25) is arranged to feed back said first and secondfeedback signals as said multiplexed feedback signals.
 37. A receiveraccording to claim 36, wherein said feedback means (24, 25) is arrangedto alternately feed back said first and second feedback signals, whereina quantization of the feedback information is based on the latestchannel estimate and an available one of said first and secondconstellation.
 38. A receiver according to claim 36, wherein saidgenerating means (24) is arranged to generate said first feedback signalbased on said channel estimation and said second feedback signal basedon a rotation of said channel estimation by a predetermined angle.
 39. Areceiver according into claim 36, wherein said generating means (24) isarranged to generate said first feedback signal based on a real part ofsaid feedback information, and said second feedback signal based on animaginary part of said feedback information.
 40. A receiver according toclaim 36, wherein said extracting means (22) is arranged to alternatelyextract a probing signal corresponding to a first beam and a probingsignal corresponding to a second beam, and said generating means (24) isarranged to alternately generate said first feedback signal based on achannel estimate for said first beam, and said second feedback signalbased on a channel estimate for said second beam.